Arrange the following in the order of property indicated for each set:
$(i)$ $F_{2}, Cl_{2}, Br_{2}, I_{2}$ - increasing bond dissociation enthalpy.
$(ii)$ $HF, HCl, HBr, HI$ - increasing acid strength.
$(iii)$ $NH_{3}, PH_{3}, AsH_{3}, SbH_{3}, BiH_{3}$ - increasing base strength.

(A) $(i)$ Bond dissociation energy usually decreases on moving down a group as the atomic size increases. However,the bond dissociation energy of $F_{2}$ is lower than that of $Cl_{2}$ and $Br_{2}$ due to the small atomic size and interelectronic repulsions in fluorine. Thus,the increasing order is: $I_{2} < F_{2} < Br_{2} < Cl_{2}$.
$(ii)$ The bond dissociation energy of $H-X$ molecules decreases as the atomic size of $X$ increases. Since the $H-I$ bond is the weakest,$HI$ is the strongest acid. The increasing order is: $HF < HCl < HBr < HI$.
$(iii)$ On moving from nitrogen to bismuth,the atomic size increases and the electron density on the central atom decreases,reducing the availability of the lone pair. Thus,the basic strength decreases. The increasing order is: $BiH_{3} < SbH_{3} < AsH_{3} < PH_{3} < NH_{3}$.